The invention relates to a method of and equipment for qualifying shock absorbers of a motor vehicle. In the course of realizing the proposed method a pair of wheels from the chassis of a vehicle are placed on a support in a known manner, and the static contact force acting in standstill is measured. The wheels are then given a constant-amplitude sinusoidal or approximately sinusoidal excitation to bring them into a vibration (swinging movement) above the natural resonance frequency of the vehicle suspension. Then, after removing the excitation, the contact force is measured as a function of the changing frequency. The equipment comprises a supporting member to transfer the vibration to at least one of the wheels, a dynamometer unit is coupled to the supporting member and an electronic measuring unit is connected to receive the signals from the dynamometer. The method and equipment complying with the invention are intended for use in qualifying the suspension system of motor vehicles, especially in the course of their official testing, thereby ensuring a substantial reduction in the time required for obtaining highly reliable accurate measuring results on which acceptable qualification can be based also for official use.
The shock absorbers mounted into motor vehicles serve, on the one hand, for damping vibrations of the vehicle chassis resulting from the unevenness of roads, and for eliminating the occurrence of spring deflections to a full bump by transforming them into slow lower-amplitude spring vibrations. On the other hand, it is also the task of shock absorbers to ensure optimum roadholding properties to satisfy road safety considerations. Roadholding properties obviously depend on the prevailing load acting on the wheels and on the friction coefficient which is a characteristic of the material of the road surface and the tires. The forces acting on the vehicle wheels are subject to constant changes depending on the unevenness of the roadway and other excitation effects. This load variation of the vehicle wheels is decisively influenced by the quality of the shock absorber. Because of the damping of the wheel swings through the dynamic load fluctuations on the wheels, and consequently, it will through the varying magnitude of the friction force of the tires, considerably affect the braking and accelerating conditions of the vehicle and its behaviour under the action of side forces.
Thus, the qualification of shock absorbers assembled into vehicles is a task of utmost importance from the point of view of road safety, and several different methods of qualification are known. A common feature of these methods, is based on the measurement of chassis swings or of the damping of wheel swings, correspondingly, the chassis or wheel is brought into vibration, and the displacements are recorded. However, in regards to road safety considerations, no satisfactory results have been obtained by the method of swinging the chassis.
In qualifying shock absorbers, according to the Recommendation TS-02-76 issued by EUSAMA (European Shock Absorber Manufacturers Association) the following main requirements have to be satisfied:
the vertical static contact force between tire and support member should be measured, PA1 the supporting member should be given a sinusoidal excitation to bring the vehicle into vibration, PA1 the ratio of the minimum dynamic contact force F.sub.min measured on the supporing member at the wheel resonance frequency to the static contact force F.sub.stat should be expressed as a percentage, this being the test result according to the formula ##EQU1## the proposed minimum frequency of exciting the supporting member is 24 Hz, one operator (driver) sitting in the driver's seat should be able to perform the measurement, including the putting of the vehicle into desired position, PA1 when a test result of up to 20% is obtained, the shock absorption is unsatisfactory, in the range of 20 to 40% the shock absorption is considered as fair, above 40% it is good, and it is excellent in the range over 60%.
For performing said qualification proceeding an arrangement is known from technical literature as a "Bogeian shock absorber tester" where a supporting member, a wheel fitted out with a tire placed on it and the chassis are brought into vibration by the driving motor through an eccentric and a spring. First, the frequency of excitation is increased into the range beyond the resonance frequency of the chassis (vehicle suspension), then, after disconnecting the driving motor, the excitation frequency is gradually reduced. In the course of this reduction of frequency, the frequency of excitation passes through the natural resonance frequency of the chassis. During runout, the displacements of the tire support is recorded on a time-scale chart.
In this case, however, the state of the vibrating system fails to correspond to the realistic road conditions, considering the dependence of the resultant resonance frequency of the rigidity of the spring of the test bench, on the mass of the supporting member, on the unsprung mass of the chassis and on the spring of the vehicle body. Consequently, the vibrating system gets detuned, so the magnitude of the measured resonance amplitude and the qualification of the shock absorber from the point of view of road safety is only possible in a circumstantial way, through correlation. A further deficiency of this approach is the necessity of knowing certain limiting values depending on the construction of the vehicles and the shock absorbers. This renders the handling of the equipment extremely cumbersome, imposing a difficult task on the operating personnel.
A test method and equipment approximately complying with the EUSAMA recommendations are described in the specification of the Hungarian Pat. No. 176 823 and in the corresponding European patent application open-laid under No. 0 049 303 /the Applicant is the Kozlekedestudomanyi Intezet--Institute for Communication Sciences--Budapest/. The method as disclosed essentially consists of establishing the mean values and the minimum tire-force pertaining to the passage through the natural frequency of the chassis, which are then used as a basis for qualifying shock absorbers. As a main feature of the equipment, the exciting unit and the supporting member are coupled together through a double-arm lever and a pre-sensing unit is linked up with the fulcrum of said lever, the unit being connected to an electronic dynamometer unit.
The main disadvantages of this arrangement and of an Italian equipment CEMB Diam 183 operating on a similar principle are as follows:
With both arrangements the first result is of higher numerical value than those obtained with the second and third readings. After the third reading the test results become stable. The reason of these deviations between successive readings lies in the increasing temperature of the oil filling in the shock absorber, that stabilizes only after the third reading. Therefore an accurate and reproducible measurement is rather time consuming.
Any change in the static wheel-load directly influences the test result. Therefore, the lower the dead weight of the vehicle the impending result will be more dependent on the vehicle load.
The vehicle must stand on a defined part of the measuring device. The required position of the vehicle on said surface can be checked by the driver either with the use of some outside assistance, or he has to get out of the driver's seat and climb into it again.
The test results depend on the tire pressure, so the air pressure in the tire has to be kept within specified limits during the testing of a shock absorber. When a row of vehicles is queing up for road safety testing, any deviation is objectionable, because--for quick passing through--each single test must be completed within a given duration of time. Should an adjustment of tire pressure be required on a vehicle before the test, this would mean taking essentially longer time than any other test, such as a brake test, headlamp setting, gas analysis, etc.